Two vehicle powertrain configurations are rear wheel drive (RWD) and front wheel drive (FWD). With additional hardware, both of these configurations may be configured to direct power to all four wheels. Because traction at any particular wheel may be limited at certain times, the ability to direct power to all four wheels may improve mobility. However, the additional hardware may introduce additional parasitic losses which may increase fuel consumption in conditions that do not require the additional capability.
In a RWD configuration, the engine may be oriented longitudinally in the vehicle such that the crankshaft axis is aligned with the direction of vehicle movement. A transmission mounted to the engine drives a rear driveshaft at a speed which may be less than or greater than the speed of the engine crankshaft. The rear driveshaft is connected to a rear axle that changes the axis of rotation, reduces the rotational speed, and drives left and right rear wheels via half-shafts while permitting slight speed differences between the wheels as the vehicle turns a corner. A RWD configuration may be adapted to also drive the front wheels by adding a transfer case between the transmission and the rear driveshaft. In addition to driving the rear driveshaft, the transfer case drives a front driveshaft that, in turn, drives a front axle. Some transfer cases include a planetary gear set that divides the torque between front and rear driveshafts while allowing slight speed differences. Other transfer cases have an actively controlled torque on demand (TOD) clutch that only drives the front driveshaft in certain conditions, such as when a controller senses loss of traction of the rear wheels.
In a FWD configuration, the engine may be oriented transversely in the vehicle such that the crankshaft axis is aligned with the axis of wheel rotation. A transmission mounted to the engine drives a front differential at a suitable speed. The front differential may be integrated into a common housing with the transmission gearbox. The front differential drives left and right front half-shafts while permitting slight speed differences between the half-shafts as the vehicle turns a corner. A FWD configuration may be adapted to also drive the rear wheels by adding a power take-off unit (PTU) that drives a rear driveshaft at a speed proportional to the speed of the front differential. A rear drive unit (RDU) may include a TOD clutch that, when engaged, drives a rear axle that, in turn, drives left and right rear half-shafts.